The present invention relates to a method for anodizing silicon substrates and a method for manufacturing acceleration sensors using the anodization method.
In recent years, acceleration sensors have been used in controlling devices such as ABS (antilock brake system), airbag systems, and suspension control systems. A surface-type acceleration sensor is known as a kind of acceleration sensor. The surface-type acceleration sensor includes a silicon substrate, a displaceable mass portion formed on the upper surface of the substrate, and a deformation gauge formed on the surface of the mass portion. In recent years, anodization technologies have been used to form the mass portion. With reference to FIGS. 9-11, a method for manufacturing a conventional surface-type acceleration sensor 41 using anodization will now be described.
FIG. 9 shows a p-type single crystal silicon substrate 42, which is anodized. On a predetermined area of the upper surface of the p-type single crystal silicon substrate 42, a p+ silicon embedded layer 43 is formed. A first epitaxial growth layer 44, which is made of n-type silicon, is laminated on the substrate 42. p+ silicon diffusion layers 45 are embedded in predetermined areas of the first epitaxial growth layer 44. A second epitaxial growth layer 46, which is made of n-type silicon, is laminated on the first epitaxial growth layer 44. On a predetermined area of the second epitaxial growth layer 46, p+ silicon diffusion layers 47 are formed and are exposed to the exterior. On the second epitaxial growth layer 46, oxide film 48, wiring patterns 49, passivation film 50, and metal protection film 51 are formed. A deformation gauge (not shown) is also formed in the second epitaxial growth layer 46. An oxide film 52 and an electrode layer 53 are laminated in that order on the bottom surface of the substrate 42. The electrode layer 53 is electrically connected to the substrate 42 through a connection opening 54.
To perform anodization, the anode of a DC power supply 55 is connected to the electrode layer 53, and the cathode is connected to a counter electrode (not shown). In this state, the substrate 42 and the counter electrode are immersed in a hydrofluoric acid solution. Then, a direct current I flows from the lower side of the substrate 42 to the upper side, which selectively becomes porous. During anodization, mainly the embedded layer 43, the diffusion layer 45, and the diffusion layer 47 are changed into porous silicon layer 56 (see FIG. 10).
Then, the porous silicon layer 56 is selectively dissolved and removed by etching using alkaline etchant, which makes the substrate 42, which includes the layers 44, 46, hollow. As a result, mass portions 57 are formed on the epitaxial growth layers 44, 46, which form the acceleration sensor 41 as shown in FIG. 11.
However, in the conventional method, the direct current is applied to an area outside the area designated for anodization. This slows the anodization. Accordingly, a more efficient anodization method has been requested.
Also, in the conventional method, the range that becomes porous may be wider than the designated area. This increases the amount of the porous part that is removed by the etching and reduces the size of the mass portions 57. In this way, the amount of wasteful removal increases and it is difficult to form a large mass portion 57. Therefore, it is difficult to produce a highly sensitive surface-type acceleration sensor using the anodization method.
Further, in the conventional method, holes may be formed in the side surfaces when the porous part is expanded to the periphery of the substrate 42. To avoid this, the size of substrate must be increased, which prevents making the acceleration sensors compact.
To solve the above problems, an objective of the present invention is to provide an anodization method for silicon substrates that efficiently makes a designated area porous.
Another objective of the present invention is to provide a method for manufacturing compact and highly sensitive surface-type acceleration sensors.
To achieve the above objectives, the present invention provides a method for anodizing a silicon substrate comprising: providing a p-type single crystal silicon substrate; forming an n-type silicon embedded layer made of n-type silicon on a predetermined area of a first surface of the p-type single crystal silicon layer, wherein an opening for permitting a current to flow is formed in the center of the n-type silicon embedded layer; forming an n-type silicon layer on the first surface of the p-type single crystal silicon substrate and on the n-type silicon embedded layer; forming a silicon diffusion layer containing a high-concentration p-type impurity on a predetermined area of the n-type silicon layer, wherein the silicon diffusion layer contacts at least the n-type silicon embedded layer in the vicinity of the interface between the p-type single crystal silicon substrate and the n-type silicon embedded layer; forming an electrode layer on a second surface, which is on the opposite side of the p-type silicon substrate from the first surface; connecting the anode of a DC power source to the electrode layer and connecting the cathode to a counter electrode, which is opposed to the p-type silicon substrate; and concentrating a current flow to an area corresponding to the opening of the n-type silicon layer in a direction from the second surface of the p-type single crystal silicon substrate toward the first surface, and advancing porosity formation in the area from the first surface toward the second surface.
In the present invention, a direct current is intensely applied to an area corresponding to an opening of the p-type single crystal silicon substrate during the anodization. Accordingly, the current is efficiently applied to the designated area, which increases the anodization speed and prevents the area outside the designated area from becoming porous.
The present invention also provides a method for manufacturing a surface-type acceleration sensor having a displaceable mass portion formed on an upper surface of a silicon substrate and a deformation gauge formed on the upper surface of the mass portion, the method comprising: providing a p-type single crystal silicon substrate; forming an n-type silicon embedded layer made of n-type silicon on a predetermined area of a first surface of the p-type single crystal silicon layer, wherein an opening for permitting a current to flow is formed in the center of the n-type silicon embedded layer; forming an n-type silicon layer on the first surface of the p-type single crystal silicon substrate and on the n-type silicon embedded layer; forming a silicon diffusion layer containing a high-concentration p-type impurity on a predetermined area of the n-type silicon layer, wherein the silicon diffusion layer contacts at least the n-type silicon embedded layer in the vicinity of the interface between the p-type single crystal silicon substrate and the n-type silicon embedded layer; forming the deformation gauge on the n-type silicon layer; forming a wiring over the n-type silicon layer; forming an electrode layer on a second surface, which is on the opposite side of the p-type silicon substrate from the first surface; connecting the anode of a DC power source to the electrode layer and connecting the cathode to a counter electrode, which is opposed to the p-type silicon substrate; concentrating a current flow to an area corresponding to the opening of the n-type silicon layer in a direction from the second surface of the p-type single crystal silicon substrate toward the first surface, and changing the area into a porous silicon layer from the first surface toward the second surface; and forming the mass portion by dissolving and removing the porous silicon layer using alkali etching.
A semiconductor device preferable for anodization for making silicon porous comprises: a p-type single crystal silicon substrate; an n-type silicon embedded layer, which is made of n-type silicon and is formed on a predetermined area of a first surface of the p-type single crystal silicon substrate; an opening, which is located in the center of the n-type silicon embedded layer to permit a flow of current; an n-type silicon layer, which is formed on the first surface of the p-type single crystal silicon substrate and on the n-type silicon embedded layer; a silicon diffusion layer, which is formed in a predetermined area of the n-type silicon layer in the vicinity of the interface between the p-type single crystal silicon substrate and the n-type silicon layer to contact at least the n-type silicon embedded layer; and a high-concentration p-type impurity, which is contained in the silicon diffusion layer; and an electrode layer, which is formed on a second surface that is located on the opposite side of the p-type silicon substrate from the first surface.